1. Field of the Invention
The present invention relates to methods for fabricating r.f. apparatus of the coupled cavity traveling wave tube type. More particularly, this invention pertains to a method for bonding thermally-mismatched elements whereby temperature-related stressing and impedance mismatches are reduced.
2. Description of the Prior Art
The traveling wave tube is a vacuum device which serves as an amplifier of microwave frequency energy. It relies upon the energy interaction that can occur between an electron beam and a microwave frequency signal. The microwave signal propagates along a slow wave structure that causes it to traverse an extended distance between two axially spaced points. This reduces the effective lateral propagation velocity from that of light to that of the electron beam velocity and transfers energy from the beam to the signal. By lowering the propagation velocity, an energy coupling is caused to take place between the beam and the microwave signal that amplifies the microwave frequency energy.
The conventional coupled cavity type traveling wave tube comprises an arrangement of interconnected cells that are serially disposed and adjacent one another along a common axis. A plurality of axially aligned passages through the cavities permits passage of the beam and each interaction cavity is coupled to an adjacent cavity by means of a coupling aperture in an endwall. Conventionally, the coupling apertures between adjacent cavities are alternately disposed on opposite sides of the electron beam axis. An electron gun containing a cathode is located within the tube for furnishing a source of electrons that are formed into a beam and directed along a straight path through the cavity passages. The electromagnetic interaction occurs along the electron beam and the microwave signal appearing at the cavity proximate the beam.
The beam is confined or focussed to the axial path by magnetic means to minimize spreading. So-called pole pieces define the cavities and walls of the slow wave structure while magnets positioned outside the vacuum region of the tube provide the magnetic flux. Protruding ferrules project from the front and back sides of the pole piece walls, serving to surround the electron beam passage and providing a concentrated, axially-extending magnetic field between the ferrule of one pole piece and that of an adjacent pole piece. The beam passage formed in the pole piece between the ends of the ferrules functions as a drift tube region.
In addition to the above-described structure, a common tube structure also includes one or more termination pieces for absorbing spurious microwave signal energy. Such termination pieces, formed of an appropriate ceramic material such as aluminum nitride or beryllium oxide impregnated with silicon carbide eliminate undesired signal reflection in the tube that result from beam-cavity interaction and from passive devices coupled to the input and output ends thereof. Such element(s) are located within a termination cavity that can include metallic elements such as sever and termination pole pieces. Depending upon the type of tube employed, the pole pieces may be of either iron (magnetic) or copper (non-magnetic) composition in accordance with the chosen mechanism for focussing the electron beam. In either case, significant problems of both an operational and a mechanical nature can arise as a result of heating due to the absorption of r.f. energy by the ceramic termination piece. It has been observed that the characteristic impedance of the termination piece is altered at high temperatures. This can result in a mismatch with the rest of the tube at elevated temperatures which will degrade effective operation. Undesired reflection of r.f. energy can cause the tube to oscillate, thereby significantly degrading its utility in applications that require precision switching. Tube designs therefore seek to contact the ceramic termination with metallic elements that function in part as heat sinks.
Accordingly, the ceramic termination is bonded to metallic elements such as pole pieces and spacers in numerous tube arrangements. In the prior art, such bonds have been achieved by sintering or brazing the elements directly to one another. The resulting so-called diffusion-type bonds are subject to failure when exposed to the significantly elevated temperatures often encountered during tube operation due to the significantly different thermal expansion coefficients of the interfacing materials. The thermally induced stresses that can occur at the bonding interface can result, for example, in fracture of the ceramic terminations which can degrade the tube's operation by reflecting the r.f. energy. Fractured chips may also fall into the electron beam hole, causing defocussing and excessive gas. In addition, other elements of the conventional traveling wave tube may require the bonding of materials of distinctly differing thermal expansion characteristics and, as mentioned earlier, in view of the high power levels often handled, such bonds can be subjected to thermal stressing that may result in device failure.